1. Field of the Invention
The present invention relates to a polarization method of a multi-layered piezoelectric body used for a filter of a portable telephone, or other suitable electronic component, and more particularly, the present invention relates to a polarization method of a multi-layered piezoelectric body in which a plurality of piezoelectric layers and a plurality of internal electrodes are alternately laminated and adjacent piezoelectric layers are polarized in the thickness direction such that the polarization directions thereof are opposite to each other.
2. Description of the Related Art
Conventionally, a length mode piezoelectric resonator has large design freedom, small spurious vibrations, and the difference df between a resonance frequency and an anti-resonance frequency is large. See, for example, Unexamined Japanese Patent Publication No. 10-4330 gazette.
FIG. 1 shows an example of this length mode piezoelectric resonator 10. The piezoelectric resonator 10 includes a base 11 in which a plurality of piezoelectric layers 12 and a plurality of internal electrodes 13 are laminated alternately. The piezoelectric layers on both sides of the internal electrodes 13 are polarized in opposite directions. Insulating films 14 and 15 are alternately provided to cover ends of the internal electrodes 13. Furthermore, external electrodes 16 and 17 are provided on opposing surfaces of the piezoelectric resonator 10. Therefore, the external electrodes 16 and 17 are alternately connected to every other one of the internal electrodes 13.
In the piezoelectric resonator 10, the polarization degree of the piezoelectric layer 12 greatly influences the properties thereof. Therefore, variations in the polarization degree within each element and variations in the polarization degree variation between elements must be minimized.
In this type of piezoelectric resonator, a block-like multi-layered piezoelectric body is provided. After polarization is performed, the piezoelectric body is cut into separate piezoelectric resonators. The polarization process of the multi-layered piezoelectric body is performed by the method shown in FIG. 2. A multi-layered piezoelectric body 1 is defined by a block-like piezoelectric ceramic material member. Here, although four piezoelectric layers 1a to 1d are shown to simplify the explanation, many layers are laminated together to produce the piezoelectric resonator. Between the piezoelectric layers 1a to 1d, internal electrodes 2a to 2c are provided. The internal electrodes 2a to 2c are alternately led out to a side surface of the piezoelectric body 1, and are connected with side surface electrodes 3 and 4. Also, by applying the DC electric field between the side surface electrodes 3 and 4, as illustrated by arrow P, the piezoelectric layers 1b and 1c on both sides of the internal electrode 2b are polarized in opposite directions thereby obtaining a desired polarization degree.
However, in the method as shown in FIG. 2, because an electric field concentrates on the edge portion of the internal electrodes 2a to 2c, the polarization degree distribution is not uniform. FIG. 3 shows an example of the polarization degree distribution in one piezoelectric layer. An oblique line illustrates the polarization degree. As shown in the FIG. 3, if the electric field is applied in the thickness direction to the piezoelectric body 1, the polarization degree at the four corner sections of the piezoelectric body 1 is substantially increased (concave distribution), and a uniform polarization degree distribution is not obtained. As a result, when lamination elements used to form the block having piezoelectric layers with non-uniform polarization degree distributions are laminated, and the lamination is cut into a rectangular shape to define an element, it is impossible to use the peripheral sections of the piezoelectric body, and thus the yield of the piezoelectric body is greatly reduced.
For example, when performing the polarization of a multi-layered piezoelectric body for series resonators (fr=450 kHz, df=55 kHz) used for a ladder-type filter by the method shown in FIG. 2, the variation in the polarization degree df in the block is at least 10 kHz. Therefore, only elements cut from near the center of the block can be used, and the polarization of the peripheral element of the block is defective and cannot be used.
Consequently, the inventions of the present application have suggested a method in which an electric field is applied to opposing external electrodes of a main surface of a multi-layered piezoelectric body, after performing the polarization (initial polarization) in one thickness direction of the multi-layered piezoelectric body, a side surface electrode which leads an internal electrode out is alternately provided. An electric field is applied between the side electrodes and only the direction of polarization of the piezoelectric layer of one side of the internal electrode is reversed (polarization reversal), and a desired polarization degree is obtained. See, for example, Japanese Unexamined Patent Application No. 2000-52743. In this method, as shown in FIG. 4, even when there is a variation in polarization degree xcex94P1 between a peripheral section and a center section in the initial polarization, when the electric field is applied in the opposite direction and the direction of polarization is reversed, polarization degree variations are reduced to xcex94P2. Thus, the non-uniformity of the polarization degree distribution after an initial polarization is corrected.
However, when the polarization of a saturated polarization degree Pmax of the piezoelectric layer having a direction of polarization that is reversed is performed until it becomes almost equal to saturated polarization degree Pmax at the time of an initial polarization, although the polarization degree variation is reduced, a polarization degree distribution of the piezoelectric layer in which polarization reversal is performed becomes concave in a similar manner as before the polarization reversal. Therefore, when a multi-layered piezoelectric body is constructed in which the piezoelectric layers are polarized in a reverse direction are laminated alternately by the above-mentioned method, the piezoelectric layer with a concave distribution in which the polarization reversal is performed and the piezoelectric layer with a concave distribution in which the polarization reversal is not performed are laminated alternately. Also, in a multi-layered piezoelectric body as a whole, a uniform polarization degree distribution is not reliably obtained.
To overcome the above-described problems, preferred embodiments of the present invention provide a polarization method for a multi-layered piezoelectric body that produces a polarization degree distribution of the entire multi-layered piezoelectric body that is uniform, thus greatly improving the yield.
According to a preferred embodiment of the present invention, a polarization method for a multi-layered piezoelectric body includes alternately laminating a plurality of piezoelectric layers and a plurality of internal electrodes and polarization of the adjacent piezoelectric layers is performed in the thickness direction thereof so as to polarize the adjacent piezoelectric layers in opposite directions, a first polarization process in which an electric field in one thickness direction is applied to the multi-layered piezoelectric body and uniformly performs the polarization in the thickness direction, and a secondary polarization process in which the electric field is applied in the opposite thickness direction to the piezoelectric layers on both sides of the internal electrodes and only the direction of polarization of the piezoelectric layer of one side of the internal electrode is reversed. The above secondary polarization is performed in a range such that a remaining polarization degree Pr2 that exists after the secondary polarization in the piezoelectric layer having a direction of polarization that is reversed does not exceed a remaining polarization degree Pr1 that exists after the first polarization.
Further, according to a preferred embodiment of the present invention, in a polarization method of the multi-layered piezoelectric body in which a plurality of piezoelectric layers and a plurality of internal electrodes are laminated alternately and the polarization of the adjacent piezoelectric layers is performed in opposite thickness directions, a first polarization process is performed in which electric fields of opposite directions are applied to the piezoelectric layers on both sides of the internal electrode and the polarization is performed on the piezoelectric layers on both sides of the internal electrode in opposite directions, and a secondary polarization process is provided in which an electric field in the opposite direction of the electric field in the first polarization process is applied in opposite directions and the direction of polarization of the piezoelectric layers of both sides of the internal electrode is reversed, wherein the above secondary polarization is performed in a range such that the remaining polarization degree Pr2 that exists after the secondary polarization in the piezoelectric layer having a direction of polarization that is reversed does not exceed the remaining polarization degree of Pr1 that exists after the first polarization.
A first polarization is performed by applying an electric field in one thickness direction to a multi-layered piezoelectric body, to uniformly polarize the multi-layered piezoelectric body in the thickness direction. Next, a secondary polarization is performed by applying an electric field in an opposite direction relative to the piezoelectric layers on both sides of the internal electrode and only the direction of polarization of the piezoelectric layer of one side of the internal electrode is reversed.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.